ISDN standard Q.931 is intended to provide an international standard to control the flow of data and signaling information between telecommunication switching systems, packet switching systems, and terminals. That standard is an effort to establish an international set of ISDN messages with each message having a number of information elements. The message format is to group information elements by codesets and to identify each information element by a codepoint. The codeset is a term that refers to a group of 133 information element identifiers. In the ISDN message structure, there are eight possible codesets, numbered 0 through 7. Codeset 0 is the current set of information elements defined by ISDN standard Q.931. Codesets 1 through 4 are reserved for future ISDN standard Q.931 expansion. Similarly, Codeset 5 is for National use; codeset 6 is for information elements specific to the local serving network; and codeset 7 is for user-specific information elements. Information elements represent submessages within the ISDN message. The code point is an numeric value assigned to an information element. That numeric value allows identification of the information element.
Although ISDN standard Q.931 exists as a standard, it is not followed with complete compliance for the following reasons. First, from country to country, differences are found in the type of information assigned to each codeset as well as the meaning of an individual codepoint within in a given codeset due to National government regulations. Because of those differences, the usefulness of ISDN Q.931 is limited for international calls. Second, within a given country (e.g. the United States), the lack of national regulations has resulted in individual corporations creating their own standards for various codesets and codepoints. As a result, within a particular country, systems and terminals manufactured by different manufacturers may not be able to communicate with each other using all of the available codesets. Finally, there may be incompatibilities within one manufacturer's product line due to the manufacturer attempting to align his products with an evolving ISDN Q.931 standard. For example, certain corporate standards originally placed certain user-specific and local serving network information elements in codeset 7. Later, these elements were redefined as part of codeset 6 by changes to ISDN Q.931. This evolution resulted in older members of a product line being incompatible with newer members of the same product line.
Since incompatibility can exist in the codesets and codepoints, a system or terminal may receive a message which contains an unknown information element. The ISDN Q.931 standard suggest two methods for handling an unknown information element. The first method is to ignore the unknown information element completely, and the second method is to drop the call for which the unknown information element was received. The problem with these two methods is that no provision is made for incompatibilities in implementations of the ISDN Q.931 standard at the national, manufacturer, or intra-manufacturer level. What is needed is a procedure that will resolve incompatibilities at those levels.